1. Technical Field to which the Invention Belongs
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereafter referred to as TFT), and to a method of manufacturing thereof. For example, the present invention relates to an electro-optical device, typically a liquid crystal display panel, and to electronic equipment loaded with this type of electro-optical device as a part.
2. Prior Art
In recent years, techniques of structuring a thin film transistor (TFT) by using a semiconductor thin film (with a thickness on the order of several nm to several hundred of nm) formed on a substrate having an insulating surface have been in the spotlight. The thin film transistor is being widely applied in an electronic device such as an IC or an electro-optical device, and in particular, its development as a switching element of an image display device has been proceeding rapidly.
Conventionally, a liquid crystal display device is known as an image display device. Active matrix type liquid crystal display devices have come into widespread due to the fact that, compared to passive type liquid crystal display devices, a higher definition image can be obtained. By driving pixel electrodes arranged in a matrix state in the active matrix type liquid crystal display device, a display pattern is formed on a screen. In more detail, by applying a voltage between a selected pixel electrode and an opposing electrode corresponding to the pixel electrode, optical modulation of a liquid crystal layer arranged between the pixel electrode and the opposing electrode is performed, and the optical modulation is recognized as a display pattern by an observer.
The use of this type of active matrix type electro-optical device is spreading, and along with making the screen size larger, demands for higher definition, higher aperture ratio, and higher reliability are increasing. Further, at the same time, demands are increasing for improving productivity and lowering costs.
Conventionally, a TN mode oriented with a 90° twist between the direction of light incident to a liquid crystal molecules and the direction of light emitted from the liquid crystal molecules is generally used as an orientation mode of a liquid crystal layer used by a transmitting type liquid crystal display device.
When manufacturing the TN mode liquid crystal display device, an orientation film is formed on one substrate and on another substrate, and a process such as a rubbing process is performed in order to set the orientation direction of the liquid crystal. These substrates are then put together such that the rubbing directions of the substrates are perpendicular to each other. By injecting a liquid crystal material, in which a chiral material for determining the twist rotation direction has been mixed in, between the pair of substrates, a liquid crystal display device having a preset twist direction is formed.
At this point, the major axis of the liquid crystal molecules is arranged parallel with respect to the substrate surface in order to have the energetically most stable arrangement, and depending upon the rubbing conditions and orientation film material, the liquid crystals are arranged possessing an angle from several degrees to approximately 10° with respect to the substrate surface.
This angle is referred to as pre-tilt angle, and by maintaining this pre-tilt angle, change of the arrangement occurs by a predetermined lining up of an edge portion in both edge portions of the major axes of the liquid crystal molecules when an electric field is applied. The orientation thus becomes continuous during operation, and an orientation defect referred to as reverse tilt domain during display can be prevented.
However, with the above TN mode, the contrast characteristics deteriorate extremely outside a specific viewing range, and a problem of a phenomenon referred to as reverse gradation develops.
This is because light having different optical modulation is seen due to: changes in arrangement, in which the orientation state of the liquid crystal molecules becomes vertical with respect to the substrate surface due to the electric field; and changes in the light advancement distance within the liquid crystal layer, and changes in the index of refraction of the light during transmission, depending upon the viewing angle and position at which an observer watches the liquid crystal display device.
Further, the liquid crystal molecules near the interface with the substrate receive strongly regulated orientation with this mode, and the initial orientation state is nearly maintained. Therefore, even if a very high liquid crystal saturation voltage (5V or more) is applied, the liquid crystal molecules in this neighborhood will not become vertical.
These are considered the primary factors causing the narrowing of the field of view characteristics of the TN mode.
In addition, a perpendicular orientation type liquid crystal mode is known as another liquid crystal display mode. The perpendicular orientation type liquid crystal mode is an orientation mode in which the initial orientation of the liquid crystals is vertical with respect to the substrate. An n-type liquid crystal material possessing negative dielectric anisotropy is used in this mode. Display is realized for this mode as well by applying an electric field between electrodes formed on the substrates.
However, because this is a mode which utilizes the double refraction of the liquid crystal, a small amount of dispersion in the pre-tilt angle is conspicuous as a dispersion in the amount of light transmitted or in the amount of light reflected. Small differences in the contact of the brush tip during the rubbing process become a cause of wavy display, which easily becomes a problem.
Further, the rubbing process itself is a process of rubbing the surface of the orientation film on the substrate with a soft hairs, and therefore this becomes a source of dust contamination. In addition, it is necessary to have sufficient counter measures against stress and deterioration of the elements on the substrate which accompanies the generation of static electricity.
Therefore, a method of orienting the liquid crystals and realizing a uniform orientation without performing the rubbing process has generally been searched for. For example, a means of manufacturing a liquid crystal display device is known in which a structure is formed on the substrate, and physical parameters such as the slope of the face of the structure which contacts the liquid crystal, the gap, and the height are regulated, and in addition, by controlling orientation together with the electric field action due to the dielectric constant of the structure. A wide angle of view equal to or greater than 160° can thus be realized by this method. However, although the conventional rubbing process becomes unnecessary with this method, complicated additional processes are required in order to orient the liquid crystal.